1. Field of the Invention
This invention relates to an electrophotographic photosensitive member used for forming images by using an electromagnetic wave, for example, ultraviolet ray, visible ray, infrared ray, X ray, gamma ray and the like, and a process for preparing the photosensitive member.
2. Description of the Prior Art
Heretofore, there have been used inorganic photoconductive materials such as Se, CdS, ZnO and the like and organic photoconductive materials such as poly-N-vinyl-carbazole, trinitrofluorenone and the like as a photoconductive material for photoconductive layers of electrophotographic photosensitive members.
However, they all suffer from various drawbacks.
For example, Se has only a narrow spectral sensitivity range, and when the spectral sensitivity is widened by incorporating Te or As, the light fatigue increases. Se, As and Te are harmful to man. When Se photoconductive layers are subjected to a continuous and repeating corona discharge, the electric properties are deteriorated. Se photoconductive layers are of poor solvent resistance. Even if the surface of an Se photoconductive layer is covered with a surface protective coating layer, the problems are not sufficiently solved.
Se photoconductive layers may be formed in an amorphous state so as to have a high dark resistance, but crystallization of Se occurs at a temperature as low as about 65.degree. C. so that the amorphous Se photoconductive layers easily crystallize during handling, for example, by ambient temperature or friction heat generated by rubbing with other members during image forming steps, and the dark resistance is lowered.
ZnO and CdS are usually mixed with and dispersed in an appropriate resinous binder. The resulting binder type photoconductive layer is so porous that it is adversely affected by humidity and its electric properties are lowered and further developers enter the layer resulting in lowering release property and cleaning property. In particular, when a liquid developer is used, the liquid developer penetrates the layer to enhance the above disadvantages, CdS is poisonous to man. ZnO binder type photoconductive layers have low photosensitivity, narrow spectral sensitivity range in the visible light region, remarkable light fatigue and slow photoresponse.
Electrophotographic photosensitive members comprising organic photoconductive materials are of low humidity resistance, low corona ion resistance, low cleaning property, low photosensitivity, narrow range of spectral sensitivity in the visible light region and the spectral sensitivity range is in a shorter wavelength region. Some of the organic photoconductive materials cause cancer.
In order to solve the above mentioned problems, the present inventors have researched amorphous silicon (hereinafter called "a-Si") and succeeded in obtaining an electrophotographic photosensitive member free from these drawbacks.
Electric and optical properties of a-Si film vary depending upon the manufacturing processes and manufacturing conditions and the reproducibility is very poor (Journal of Electrochemical Society, Vol. 116, No. 1, pp 77-81, January 1969). For example, a-Si film produced by vaccum evaporation or sputtering contains a lot of defects such as voids so that the electrical and optical properties are adversely affected to a great extent. Therefore, a-Si had not been studied for a long time. However, in 1976 success of producing p-n junction of a-Si was reported (Applied Physics Letter, Vol. 28, No. 2, pp. 105-7, 15 Jan. 1976). Since then, a-Si drew attentions of scientists. In addition, luminescence which can be only weakly observed in crystalline silicon (c-Si) can be observed at a high efficiency in a-Si and therefore, a-Si has been researched for solar cells (for example, U.S. Pat. No. 4064521.
However, a-Si developed for solar cells can not be directly used for the purpose of photoconductive layers of practical electrophotographic photosensitive members.
Solar cells take out solar energy in a form of electric current and therefore, the a-Si film should have a low dark resistance for the purpose of obtaining efficiently the electric current at a good SN ratio [photo-current (Ip)/dark current (Id)], but if the resistance is so low, the photosensitivity is lowered and the SN ratio is degraded. Therefore, the dark resistance should be 10.sup.5 -10.sup.8 ohm.cm.
However, such degree of dark resistance is so low for photoconductive layers of electrophotographic photosensitive members that such a-Si film can not be used for the photoconductive layers.
Photoconductive materials for electrophotographic apparatuses should have gamma value at a low light exposure region of nearly 1 since the incident light is a reflection light from the surface of materials to be copied and power of the light source built in electrophotographic apparatuses is usually limited.
Conventional a-Si can not satisfy the conditions necessary for electrophotographic processes.
Another report concerning a-Si discloses that when the dark resistance is increased, the photosensitivity is lowered. For example, an a-Si film having dark resistance of about 10.sup.10 ohm.cm shows a lowered photoconductive gain (photocurrent per incident photon). Therefore, conventional a-Si film can not be used for electrophotography even from this point of view.
Other various properties and conditions required for photoconductive layers of electrophotographic photosensitive member such as electrostatic characteristics, corona ion resistance, solvent resistance, light fatigue resistance, humidity resistance, heat resistance, abrasion resistance, cleaning properties and the like have not been known as for a-Si films at all.
The present inventors have succeeded in producing a-Si film suitable for electrophotography by a particular procedure as detailed below.